1. Field of the Invention
The present invention relates to a charge element which evenly charges the outer face of a photoreceptor element as a photoreceptor drum used in a copier, a facsimile machine, a printer or a combined machine as well as to a process cartridge including the charge element, and an image forming apparatus including the process cartridge.
2. Description of the Prior Art
In prior art an electrophotographic image forming apparatus such as a copier, a facsimile machine, a printer or a combined machine comprises a photoreceptor drum on which an electrostatic latent image is formed, a charge roller evenly charging the outer face of the photoreceptor drum, an optical write unit exposing the evenly charged outer face of the photoreceptor element to form an electrostatic latent image, a develop unit developing the electrostatic latent image to form a toner image, a transfer unit transferring the toner image onto a paper sheet, and a cleaning unit removing remnant toner from the outer face of the photoreceptor drum after the transfer.
A contact type charge unit is generally used, comprising a charge roller contacting with the surface of the photoreceptor drum and a power source to apply direct voltage or alternating voltage to the charge roller. In this charge unit the charge roller is applied with the direct or alternating voltage to evenly charge the photoreceptor drum at a high voltage. It is known that the charge mechanism of the charge roller is an electric discharge according to Paschen's law in a finite space between the charge roller and the photoreceptor drum.
However, the contact type charge unit has various problems that due to the contact between the charge roller and the photoreceptor drum, a material composing the charge unit may exude and attach to the surface of the photoreceptor drum or a trace of the contacting charge unit may remain thereon. Moreover, other problems are noise from a vibrating charge unit when applied with an alternating voltage, a decrease in chargeability of the charge roller due to attached toner caused by the exuding material, and permanent deformation of the charge roller when the photoreceptor drum stops operating in a long period of time. In view of solving the problems, Japanese Patent Application Publication No. 2006-162646 (Reference 1), No. 2007-121480 (Reference 2), No. 2009-271131 (Reference 3) disclose a charge unit in which a charge unit and a photoreceptor drum are arranged with a distance between them, for example.
The charge roller in this charge unit comprises a body in parallel to the photoreceptor drum, gap retainer elements at both ends of the body and whose diameter is larger than that of the body, and a bias element biasing the body to the photoreceptor drum to allow the gap retainer elements to closely contact with the surface of the photoreceptor drum. The body and the photoreceptor drum are disposed with a distance and the distance may change owing to smudge or smear on the body and the photoreceptor drum, causing unevenness in charging. In the charge unit disclosed in the above references, the photoreceptor drum is evenly charged by superposing direct voltage and high alternating voltage on the body of the charge roller.
However, there still remains a problem that the appliance of the alternating voltage to the roller body may cause contaminant as a discharge product on the photoreceptor drum to disperse between the body and the photoreceptor drum and accumulate on the surface of the body over time. This causes unevenness in resistance of the surface of the body and uneven discharge, resulting in generation of a defective image.
In order to reduce the amount of contaminant on the photoreceptor drum, the charge roller can be disposed as far as possible from the photoreceptor drum. However, this requires an increase in the alternating voltage which is likely to cause anomalous discharge due to leak current, resulting in generation of a defective image including white dots.
In the meantime, the surface layer of the above charge roller (roller body) is made of conductive materials such as Ketjen Black®EC, conductive carbon as acetylene black, conductive polymer as polyaniline, polypyrrole, polyacetylene, and inorganic ion-conducting materials as sodium perchlorate, lithium perchlorate, calcium perchlorate, lithium chloride. By use of the conductive carbon, anomalous discharge due to the leak current is likely to occur, which may likely generate defective images including white dots. By use of the ion-conducting materials, since they are dispersible (soluble) in molecule level, anomalous discharge due to the leak current is unlikely to occur, preventing generation of defective images. The surface layer of the roller body disclosed in Reference 3 is made of ion-conducting materials with low resistance so that the charge roller can exert good chargeability even with a change in the distance from the photoconductor drum.
However, the charge unit in Reference 3 still faces a problem that at the beginning of usage (hereinafter, initial usage period), the surface of the roller element gets contaminated gradually over time. The contamination on the roller surface prevents the photoreceptor drum from being properly charged and causes anomalous discharge.